Vertical variation in mass and CO₂ efflux of litter from the ground to the 40m high canopy in a Bornean tropical rainforest

Litter usually drops to the ground, but some is retained within the canopy. Mass and CO₂ efflux from retained litter may play a significant role in carbon cycling in tropical rainforests where emergent trees with complicated canopy structures are observed. This study aimed to show vertical variation in the mass and CO₂ efflux (R_litter) of the retained litter and examined its contribution to carbon cycling in a Bornean tropical rainforest. The retained litter was collected from six 40 m tall columns with a radius of 1.6 m using an 85 m tall crane at six subplots. Dry mass and R_litter were measured in the laboratory. The mass of the retained litter from the sampling points ranged from 0.35 g to 187.0 g, and R_litter of the retained litter (42.4 ± 43.4 mg CO₂ kg⁻¹ h⁻¹) was significantly lower than that of the soil surface litter (85.9 ± 39.2 mg CO₂ kg⁻¹ h⁻¹) because of the lower water content in the retained litter. The mean mass of the retained litter measured from the ground to the canopy was 40.6 ± 22.2 g m⁻². The greatest mass was observed at a height of 0–2 m. The ratio of the retained litter mass to the soil surface litter mass ranged from 1.0% to 11.5% with a mean of 5.2%, suggesting that litter retained within the canopy can contribute to carbon storage in this forest. On the other hand, R_litter did not differ with height, and the ratio of R_litter to aboveground autotrophic respiration was less than 1%. This study is the first to show that the litter retained within the canopy can sporadically be a moderate hot spot for C storage, but it is not a significant source of released CO₂.